Correct me if I'm wrong...
Base cooling with 318 is a 19" core w/shroud (looks like that's what you have).
HD cooling with 318 is a 26" core w/shroud.
Base cooling with /6 is also 19" core, but w/o shroud.
HD cooling for /6 is 22" core w/shroud.
If you buy one, make sure you get the right one.